1. Field of the Invention
The present invention relates to a light emitting device provided with optically permeable material that allows light transmission from a light emitting element.
2. Description of the Related Art
Semiconductor light emitting devices are small size, highly power efficient devices that can emit bright colors. Further, semiconductor light emitting devices, which are semiconductor devices, do not burn-out as light-bulbs do. In addition, semiconductor light emitting devices have excellent initial operating characteristics and are robust with respect to vibration and ON-OFF switching repetitions. A semiconductor light emitting device light source can be combined with wavelength converting material excited by the light source to emit different colored light. Accordingly, light emitting devices have been developed that can emit light of various colors consistent with the principles of color mixing. To take advantage of these superior characteristics, semiconductor light emitting devices such as light emitting diodes (LEDs) and laser diodes (LDs) are used as various types of light sources. In particular, light emitting devices have recently drawn attention as long-life, low power-consumption next-generation lighting that can replace fluorescent lighting. Consequently, further increase in light output and improvement in light emission efficiency is in demand. In addition, there is demand for directional light sources with superior brightness for projection lighting such as in automobile headlights.
For example, Japanese Laid-Open Patent Publication 2007-19096 proposes a light emitting device 900, and FIG. 10 shows a cross-section view of this light emitting device 900. The light emitting device 900 is made up of an LED element 901, and a case 904 that holds the LED element 901. The case 904 is open on the side where light is extracted and the LED element 901 is mounted inside the open-side. Further, the inside of the case 904 is filled with a coating material 903 that includes light reflecting particles, and the open-side surfaces of the case 904 excluding the light extracting surface of the LED element 901 are covered with this coating material.
In addition, a fluorescent material layer 902 is disposed in sheet-form on the outer surface of the coating material 903 over the light extracting surface. The fluorescent material layer 902 is formed from resin that includes fluorescent material such as Yttrium Aluminum Garnet (YAG). The fluorescent material is excited by light emitted from the LED element 901 (blue light) and re-emits light that is converted in wavelength (yellow light).
The fluorescent material layer 902 is formed to cover the entire light extracting surface of the LED element 901, and has a light emitting surface exposed on the light extracting side. As a result, primary light (blue light) from the LED element 901 is mixed with a part of the primary light that is converted in wavelength to secondary light (yellow light) to obtain white light from the light emitting surface.
However, in the type of light emitting device described above, individual structural elements can thermally deform as a result of temperature rise during operation and the thermal history during fabrication. Thermal deformation can cause the fluorescent material layer 902 to be pushed up by the coating material 903 below. Consequently, this type of light emitting device has the problem that the fluorescent material layer 902 can delaminate or fall off. Further, since a significant amount of heat is not radiated from the fluorescent material layer 902 light extracting surface, it is necessary to increase the contact surface area of the coating material 903 and fluorescent material layer 902, which form a heat conducting path. However, if the size of the fluorescent material layer 902 is increased, brightness degradation can become a problem.
Thus, it is an object of the present invention to provide a light emitting device that can emit bright light and has superior reliability.